The invention relates to a method and an apparatus for removing organic impurities from the surface of drums or belts.
Drums or belts that are used for conveying or processing web material such as paper, textiles or foils frequently have the problem that the surface of the drum or belt becomes stained with low-molecular organic compounds diffusing out of the treated web materials. For example, when paper is treated in printing presses it is known that conveyor drums, printing drums and the like are contaminated by wax that emerges from the paper. The same applies for photoconductors, fixing drums or intermediate image carriers, that are directly or indirectly brought into contact with copying paper in copying machines. Likewise, in processes for manufacturing or treating plastic foils, in particular when plastic foil is extruded from an elongated slot-type nozzle, conveyor drums or chill rolls are likely to become stained with organic compounds emerging from the freshly extruded plastic material. Since such impurities accumulate on the surface of the drum, it is necessary for a long-term proper operation of the equipment that such impurities are removed continuously or in certain intervals.
Heretofore, mechanical cleaning methods, chemical methods such as washing with solvents, and contact transfer methods or combinations of these methods have been used for this purpose.
In the contact transfer process, a cleaning drum rolls over the surface to be cleaned, and the surface material of the cleaning drum and the temperature conditions are adapted in accordance with the impurities to be removed and in accordance with the surface properties of the substrate to be cleaned, so that the impurities are transferred onto the surface of the cleaning drum by adhesion. These methods are however limited to a narrow spectrum of impurities and substrates and further have the drawback that it is relatively difficult to remove the impurities, in turn, from the surface of the cleaning drum. In another variant of the contact transfer process, a cleaning belt is used instead of a cleaning drum. Here, the same drawbacks are encountered. Although the problem to remove the impurities from the cleaning belt can in this case be eliminated by using disposable cleaning belts or wipers, the provision and the disposal of the consumable material leads to increased costs.
Mechanical cleaning methods are in many cases cumbersome and of poor efficiency and can easily lead to damage or wear of the surface to be cleaned.
Chemical methods are also relatively cumbersome in most cases, and in addition, are problematic in terms of environmental pollution because vapours of solvents are generated.
It is an object of the invention to provide a simple, efficient and widely applicable method for removing organic impurities, in particular low-molecular organic compounds, from the surface of drums or belts.
According to the invention, this object is achieved by directing a jet of an atmospheric plasma onto the surface.
U.S. Pat. No. 5,837,958 discloses a plasma nozzle capable of generating a jet of a relatively cool atmospheric plasma. This plasma nozzle is mainly used for pre-treating plastic surfaces before they are coated with adhesives or lacquers or before they are printed on, so that the surface can more easily be wetted with liquids. This pre-treatment effect is due to the fact that the atmospheric plasma contains a high concentration of chemically highly reactive ions, radicals and excited atoms and molecules which reduce the surface tension of the treated substrate. The invention is based on the discovery that such an atmospheric plasma, thanks to its high reactivity, is also suitable for chemically destroying organic impurities, in particular low-molecular organic compounds, and for transforming them into volatile compounds which will then evaporate away from the treated surface. Since the atmospheric plasma has a comparatively low temperature, comparable to the temperature of a candle flame, and since it is sufficient for destroying the organic compounds that the plasma jet sweeps over the treated surface only for a short time, the method can be employed for a large variety of substrates to be cleaned without causing damage to the surface of the substrate itself. The above-mentioned effect that the surfaced tension of the substrate is reduced by the plasma treatment, is a welcomed side-effect in certain applications.
An apparatus according to the invention for cleaning drums or belts comprises at least one plasma nozzle which generates a jet of an atmospheric plasma directed onto the surface of the drum or belt, and which, in case of a drum, can be moved over the surface of the drum in axial direction of the drum and, in case of a belt, can be moved transversely to the feed direction of the belt, so that the entire width of the surface of the drum or belt, or at least the part of the surface to be cleaned, is swept by the plasma jet.
If a drum is rotating with relatively high speed, the plasma nozzle can be moved intermittently or with low speed in axial direction of the drum, so that an annular or helical track on the drum surface is cleaned during each revolution of the drum.
In case of a drum rotating at a relatively low speed or being driven only intermittently and in case of a belt, the plasma nozzle can be oscillated with relatively high speed, so that the surface of the drum or belt is swept by the plasma jet in width direction.
In both cases it is possible, by using a plurality of plasma nozzles moved together, to reduce the distance to be travelled by the individual nozzles to a fraction of the total working width.
The plasma nozzle may also be configured to generate a divergent plasma jet which has the shape of a cone or a fan and sweeps a larger area of the surface to be cleaned. Examples for such plasma nozzles are described in the German utility models DE 299 21 694 U1 and DE 299 19 142 U1.